Reciprocating pumps, as contrasted to centrifugal pumps, are relatively efficient. There is considerably less slippage around a piston than about the rotor of a centrifugal pump. Reciprocating pumps however, are characterized by low volumetric capacity which is associated with the relatively slow reciprocation of the pistons. What is needed is a pump having the high flow rate of a centrifugal pump, but also having the high efficiency of a reciprocating pump.